This invention relates to a photoreceptor, for example, an electrophotographic photoreceptor.
In the prior art, as the electrophotographic photoreceptors, those having ZnO or CdS dispersed in a resin binder have been known. However, these photoreceptors involve problems with respect to environmental pollution, thermal stability and mechanical strength.
On the other hand, electrophotographic photoreceptors employing amorphous silicon (a-Si) as matrix have recently been proposed. a-Si has the so called dangling bonds formed by cleavage of the bondings of Si--Si, and there exist a large number of localized levels within the energy gap caused by these defects. For this reason, hopping conduction of thermally excited carrier occurs to make dark resistance smaller and also worsen the photoconductivity by trapping of the photo-excited carrier in the localized levels. Accordingly, it has been practiced to compensate the above defects with hydrogen atoms to bond Si to H, thereby embedding dangling bonds in the matrix.
Such an amorphous hydrogenated silicon (hereinafter called a-Si:H) has a resistivity in the dark place of 10.sup.8 to 10.sup.9 .OMEGA.-cm, which is about lower by 1/10000th as compared with amorphous Se. Accordingly, a photoreceptor comprising a monolayer of a-Si:H has the problems of great dark decay speed of the surface potential and low initial charging potential. However, on the other hand, when a light in the visible and infrared regions is irradiated, the resistivity can be greatly reduced and therefore it has very excellent characteristics as the photosensitive layer of a photoreceptor.
FIG. 1 shows an electrophotographic copying machine in which an a-Si type photoreceptor employing the above a-si:H is incorporated as a body material. In this copying machine, a manuscript mounting stand 3 made of glass for mounting the manuscript 2 and a platen cover 4 for covering the manuscript 2 are arranged on the upper part of the cabinet 1. Below the manuscript stand 3, an optical scanning stand comprising as first mirror unit 7 equipped with a light source 5 and a first mirror for reflection 6 is provided so as to be linearly movable in the left and right directions in the Figure, and a second mirror unit 20 for making constant the optical path between the manuscript scanning point and the photoreceptor moves corresponding to the speed of the first mirror unit, thereby permitting the reflected light from the manuscript side to be incident through the lens 21 and the mirror for reflection 8 on the photoreceptor drum 9 in shape of a slit. Around the drum 9, there are arranged a corona charger 10, a developing instrument 11, a transfer section 12, a separating section 13 and a cleaning section 14, respectively. The copying paper 18 delivered from the feed paper box 15 via the respective paper feeding rollers 16 and 17 receives the toner image by transfer from the drum 9 and further fixed at the fixing section 19 before discharged into the tray 35. At the fixing section 19, fixing operation is conducted by passing the developed copying paper between the heating roller 23 including internally a heater 22 and the pressurizing roller 24.
However, a photoreceptor having the surface of a-Si:H has not been investigated fully about the chemical stability of the surface such the influences when exposed to the air or humidity over a long term or the influences by the chemical species formed by corona discharging. For example, such a photoreceptor after being left to stand for one month or longer has been known to receive influence by himidity, whereby the receptive potential is markedly lowered. On the other hand, concerning amorphous hydrogenated silicon carbide (hereinafter called as a-SiC:H), its preparation method and existence are disclosed in "Phil. Mag. Vol. 35" (1978), etc., and its characteristics are reported to reside in heat resistance, high surface hardness, higher dark resistivity (10.sup.12 -10.sup.13 .OMEGA.-cm) as compared with a-Si:H, and variable optical energy gas over the range of 1.6 to 2.8 eV depending on the carbon content. However, there is involved the drawback that the long wavelength sensitivity is worsened due to the broadened band gap caused by inclusion of carbon.
Such an electrophotographic photoreceptor comprising a combination of a-SiC:H and a-Si:H is disclosed in, for example, Japanese Provisional Patent Publication No. 127083/1980. According to this disclosure, there is prepared a function separation type two-layer structure in which an a-Si:H layer is used as the charge generation (photoconductive) layer and an a-SiC:H layer is provided as the charge transport layer beneath the charge generation layer, with the upper layer a-Si:H attaining photosensitivity in broader wavelength region and improvement of charging potential being intended by the lower layer a-SiC:H which forms a hetero-junction with the a-Si:H layer. However, dark decay cannot sufficiently be prevented and the charging potential is still unsatisfactory to be impractical in such a photoreceptor. Moreover, existence of the a-Si:H layer on the surface will worsen chemical stability, mechanical strength, heat resistance, etc.
On the other hand, Japanese Provisional Patent Publication No. 17592/1982 discloses a photoreceptor of a function separation type three-layer structure constructed by forming a first a-SiC:H layer as the surface modifying layer on a charge generation layer comprising a-Si:H and forming a second a-Sic:H layer as the charge transport layer on the back surface (on the substrate electrode side).
Whereas, in the known photoreceptors, it has been discovered that the problems as mentioned below is involved particularly in the a-SiC:H charge transport layer.
That is, although the known a-SiC:H may be acceptable in charge transporting ability (carrier range (.mu..tau.)=mobility .times.life time) and charge retentivity (dark resistance .rho..sub.D), temperature dependency of .rho..sub.D is great, whereby the retentivity of charged potential at higher temperature will be deteriorated to the extent unacceptable in practical application. Such a defect will also ensue when the charge transport layer is constituted of an amorphous silicon nitride (a-SiN).